1. Field of the Invention
The invention lies in the field of fluid sensors. The invention relates to a circuit for a NOx measurement sensor.
In order to measure the NOx concentration in a gas, for example in the exhaust gas of an internal combustion engine, it is conventional to employ a thick-layer measurement sensor. Such a measurement sensor is described, for example, in the publication by N. Kato et al., "Thick Film ZrO.sub.2 NOx Sensor for the Measurement of Low NOx Concentration", Society of Automotive Engineers, Publication number 980170, 1989, or in N. Kato et al., "Performance of Thick Film NOx Sensor on Diesel and Gasoline Engines", Society of Automotive Engineers, publication number 970858, 1997. The measurement sensor has two measurement cells and is composed of a zirconium oxide that conducts oxygen ions. The sensor implements the following measurement concept: in a first measurement cell, to which the gas to be measured is fed through a diffusion barrier, a first oxygen concentration is established by a first oxygen ion pump current, the intention being that no decomposition of NOx takes place. In a second measurement cell, which is connected to the first cell through a diffusion barrier, a second oxygen ion pump current lowers the oxygen content further. The decomposition of NOx at a measurement electrode leads to a third oxygen ion pump current, which is a measure of the NOx concentration. The entire measurement sensor is brought to an elevated temperature, for example 750.degree. C., by an electric heater.
In order to establish the oxygen ion pump currents, the Nernst voltage is tapped off in the respective measurement cells. Digital microcontrollers are typically employed for the controllers. Two A/D ports on the A/D converter of the microcontroller are needed for one Nernst voltage because the voltage can be measured only with respect to the reference potential of the microcontroller. Accordingly, the voltage across the measurement electrode is typically measured with respect to the reference potential of the microcontroller. In addition, the voltage across the reference electrode is measured with respect to the reference potential of the microcontroller. The Nernst voltage is obtained by forming the difference between these two voltages. If an 8-bit converter and 5 V range are employed, the resolution is then 20 mV. However, the accuracy of the detection of the reference variable is inadequate in the control loop.
Therefore, microcontrollers with 10-bit converters are needed. Such microcontrollers are relatively expensive and available only in a few models. As such, the choice for the production or configuration is restricted.